refactor: adds function prototype

[baszalmstra]

Adds the following concepts:

• Function signature: just the type signature of a function
• Function prototype: a type signature and a name but not an implementation
• Function definition: a prototype and an implementation

Coupled to: mun-lang/abi-c#7 and mun-lang/runtime-ffi#6

[codecov]

Codecov Report

Merging #166 into master will decrease coverage by 0.13%.
The diff coverage is 79.77%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   82.90%   82.77%   -0.14%     
==========================================
  Files         163      163              
  Lines       11226    11239      +13     
==========================================
- Hits         9307     9303       -4     
- Misses       1919     1936      +17     

Impacted Files	Coverage Δ
crates/mun/src/main.rs	0.00% <0.00%> (ø)
crates/mun_abi/src/lib.rs	92.85% <ø> (ø)
crates/mun_runtime/tests/memory.rs	100.00% <ø> (ø)
crates/tools/src/abi.rs	91.66% <ø> (-4.17%)	⬇️
crates/mun_runtime/src/assembly.rs	62.76% <35.29%> (-2.07%)	⬇️
crates/mun_abi/src/autogen.rs	64.44% <75.00%> (-0.18%)	⬇️
crates/mun_abi/src/autogen_impl.rs	93.01% <82.89%> (-2.39%)	⬇️
crates/mun_runtime/src/lib.rs	72.04% <87.50%> (+0.16%)	⬆️
crates/mun_abi/src/function_info.rs	100.00% <100.00%> (ø)
crates/mun_codegen/src/code_gen/symbols.rs	94.07% <100.00%> (ø)
... and 7 more

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